Method for connecting a die assembly to a substrate in an integrated circuit

ABSTRACT

A semiconductor device includes a substrate, a die assembly attachable to the substrate and a flexible strip extending over the substrate and the die assembly. The flexible strip has one or more routing circuits carried thereon. The die assembly and the substrate are arranged to be electrically connected through the one or more routing circuits carried on the flexible strip.

This application is a divisional of patent application Ser. No.11/485,194, filed on Jul. 12, 2006, which is a continuation ofInternational Application No. PCT/SG2004/000011, filed Jan. 12, 2004,which designated the United States and was published in English, both ofwhich applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method for connecting a die assemblyto a substrate in an integrated circuit and a semiconductor device foruse in an integrated circuit.

BACKGROUND

In conventional integrated circuits, particularly memory chips, severaldies may be stacked on top of each other in a package to increase theoverall capacity of the device. A ball grid array (BGA) structure istypically used for the stacked-die structure device. In such devices, afirst semiconductor die is attached to the substrate via an adhesivelayer. A wire bonding process is applied to electrically connect thefirst die to the substrate. A second die is then stacked on the firstdie with an adhesive layer formed between the first die and the seconddie. A wire bonding process is then performed to electrically connectthe second die to the substrate and/or the first die. A molding compoundis then formed over the substrate to encapsulate the substrate, the diesand the wires. Finally, solder balls are attached to the under surfaceof the substrate to contact the solder pads on the substrate. In pyramidtype stacked-die applications, the first die is required to be largerthan the second die.

The effectiveness of wire bonding depends on the accuracy control of dieplacement, selection of wire loops, wire type, capillary type andprocess parameters governed by the die attachment, wire bond and moldingdesign rules. Such design rules include the control of die placement towithin ±50 μm, the highest loop height to within 150 μm±25 μm, a bondline thickness to less than 75 percent of the respective die thickness,a wire angle to within ±50° and a wire sweep to less than 10 mills (254μm). Poor settings of the process parameters and/or inappropriate wireand capillary selection may lead to a high assembly reject rate withassociated problems like wire short circuits, neck ball, failedconnections to the pads and/or leads, cratering of the pad and otherproblems.

Even with a single die device there is a problem in bonding theconnections from the dies to the substrate and such problems areincreased in multi-stack devices, particularly in wire bond/wire bondmode or flip-chip/wire bond mode. In particular, the molding process candetach the wires or bring them into contact with adjacent wires causingshort circuits. Short circuiting may also be caused by wire strays, wireleaning and wire sweep after molding or wire bonding and can affect theperformance of the device.

Conventional methods of electrically interconnecting the stacked die tothe substrate have a number of defects. In particular, conventionalmethods may suffer from one or more of the following problems, such aswire shorting due to die placement offset, inappropriate wire andcapillary use, wrong parameter settings (wire loops), mishandling ofbonded strips, excessive wire sweep after molding, non-stick-on pad dueto the wrong parameter setting (USG current), contamination of the bondpad, large probe mark size and wrong bonding.

Furthermore, wire leaning and/or wire sways immediately after wirebonding could affect the performance of molding with the highpossibility of wire shorting as a result. In addition, inappropriateloop length and height of the loops, as well as weak interconnectadhesion could all further aggravate the encapsulation process. Also, acapillary touching a wire and a wire touching the die edge could bothresult in poor bonding performance, which affects the operation of thedevice.

In view of the foregoing problems with conventional processes anddevices, a need exists for a quick and an easily applied method forconnecting dies and substrates.

SUMMARY OF THE INVENTION

In general terms, the present invention provides a means of electricallyinterconnecting one or more dies to each other and/or a substratethrough a flexible strip. Preferably, the bond pads of a second die areinterconnected to a first die and/or the substrate through the bondingof the flexible strip. The bond pads of the first die may also beconnected to the substrate through the bonding of the flexible strip.The flexible strip preferably includes routing circuits applied to anunderlying surface thereof with a series of matching solder-on padsbeing attached to the ends of the routing circuits. The flexible stripis preferably pinned down to the substrate by guided pins on analuminium clamp template. Bonding of the flexible strip onto the one ormore dies (silicon-to-silicon) and the substrate is through thesolder-on pads on the flexible strip, bond pads on the one or more diesand bond fingers on the substrate. Bonding is achieved, for example, byapplication of ultrasonic or heating techniques.

This method replaces conventional wire bonding processes. It isadvantageous as it facilitates bonding and more particularly increasesthe efficiency in chip bonding procedures.

According to a first aspect of the invention, a die assembly can beconnected to a substrate in an integrated circuit. The die assembly isattached to the substrate. A flexible strip is located over thesubstrate and the die assembly. The flexible strip carries one or moreconducting routing circuits terminated at each end with a solder-on pad.Electrical contact is established between the die assembly and thesubstrate through the one or more routing circuits carried on theflexible strip.

According to a second aspect of the invention, a semiconductor device isprovided for use in an integrated circuit. This device includes asubstrate and a die assembly attachable to the substrate. A flexiblestrip extends over the substrate and the die assembly, the flexiblestrip having one or more routing circuits carried thereon. The dieassembly and the substrate are arranged to be electrically connectedthrough the one or more routing circuits carried on the flexible strip.

Preferably, the flexible strip is formed of polyimide material and maybe in a tape and reel format or a strip format for ease of use.

According to a third aspect of the invention there is provided anintegrated circuit comprising one or more devices of the type definedabove.

According to a fourth aspect of the invention there is provided anintegrated circuit formed according to the method defined above.

One or more embodiments of the present invention offer the advantage ofreducing the waste of materials which, in conventional systems andmethods, are due to items rejected because of failures in the wirebonding process; for example, the failure of the wires to stick on bondpads and wedges, damaged wires and bonding units where the wires arebonded to the wrong pads and therefore have to be discarded. Furtheradvantages of one or more embodiments of the present invention are acost saving due to the materials used, a time saving as the bonding iscarried out in one step rather than bonding each wire separately, andmuch less skill is required from the operator when applying the methodsembodying the invention than when using conventional processes. Also,the forming of complex wire loops such as the crossing of wires,high/low loops and the different modes of bonding does not occur due tothe presence of the flexible strip.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only andwith reference to the following drawings in which:

FIG. 1 is a plan view of a stack of dies mounted on a substrateaccording to an embodiment of the present invention;

FIG. 2 is a plan view of a number of flex strips connected together foruse in tape and reel format or strip format;

FIGS. 3 a, 3 b and 3 c show side perspective views of the flex stripinterconnection process according to an embodiment of the invention; and

FIG. 4 is a perspective view of a multi-die device according to anembodiment of the invention after flex strip interconnection.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIGS. 1 and 2 show a multi-die device 1 comprising two dies 2, 3, onedie being mounted on top of the other, the top die 2 having a smallersurface area than the lower die 3. The pair of dies 2, 3 are mounted asa unit on a substrate 4. A plurality of flexible strips 6 are used toelectrically connect the contacts 8 on the dies 2, 3 to each other(die-to-die) and the contacts 8 on the dies 2, 3 to the bond fingers 10on the substrate 4. The flexible strips 6 may be formed of, for example,polyimide material.

The strips 6 may be manufactured in a variety of thicknesses dependingupon the specifications of the bonding pitch. The length and width canbe the same as the lead frame. The length and width of the polyimidestrips are designed to be the same as the lead frame, having aconductive strip width of around between 0.75 to 1 μm.

FIG. 2 shows a number of the flex strips 6. The strips 6 may have anumber of location holes 12 for ease of positioning the strip 6 relativeto the substrate 4. The strips 6 may be manufactured in tape and reelformat or in strip format to form templates for ease of manufacture andfast volume production. Each of the strips 6 contain, on the undersidethereof, a number arrays of routing circuits 13 to match the dies 2, 3and substrate 4 to which they are to be bonded. A number of solder-onpads (not shown) are located at the ends of the routing circuits 13.

The process of interconnecting the strips 6 to the dies 2, 3 and thesubstrate 4 is shown in FIGS. 3 a, 3 b and 3 c. As shown in FIG. 3 a, analuminum template 14 is connected (for example, permanently connected)to a holder 16. The holder 16 contains control and pre-heating circuitry(not shown). The aluminum template 14 is heated by the pre-heatingcircuitry in the holder 16. For example, the aluminum metal template 14can be preheated to punch the flex strips 6 to be bonded onto thesilicon-to-silicon and substrate 4. The underside of the aluminumtemplate 14 has a number of guide pins 18 formed thereon. The guide pins18 can hold the tape/reel for alignment with the die pad openings.

FIG. 3 b shows a strip 6 from a tape/reel that is suitable for bondingonto the dies 2, 3 and onto the substrate 4. The strip 6 includes thearray of routing circuits 13 and a plurality of location holes 12 forlocating the strip 6 over the substrate 4 so that the array of routingcircuits 13 is located over the substrate 4 and dies 2, 3. Punch holes20 (4nos) can be included for each unit.

FIG. 3 c shows the substrate 4 with two dies 2, 3 stacked one upon theother and attached thereto ready to receive the flex strip 6 shown inFIG. 3 b.

To assemble the device, the substrate 4 with the dies 2, 3 stackedthereon is positioned under the aluminum template 14. For example, thedies 2, 3 can be stacked upon each other and attached with glue.

The strip 6 is transported to the bonding area via conveyors and it ispositioned between the template 14 and the substrate 4. The aluminumtemplate 14 is preheated and brought down onto the strip 6 forcing thestrip 6 into contact with the substrate 4 and dies 2, 3. The guide pins18 on the template 14 are thereby also preheated and punch through thestrip 6 causing punch holes 20 to be created. The guide pins 18 andpunch holes 20 retain the strip 6 firmly in position over the substrate4 such that the solder pads on the routing circuits 13 are pressed ontothe bond pads of the dies 2, 3 and the bond fingers 10 on the substrate4.

The bond between the solder pads and the bond pads and fingers iscreated by ultrasonic or heating techniques. The template 14 is thenremoved and the device is ready for encapsulation.

FIG. 4 shows the assembled device prior to encapsulation and comprisingthe substrate 4, which may have multiple layers, for example between twoto four layers. The layers carry the stacked dies 2, 3 and the strip 6connecting the bonding pads on the dies and the bond fingers 10 on thesubstrate 4.

Various modifications to the embodiments of the present inventiondescribed above may be made. For example, in a preferred embodiment, thesubstrate may comprise two or more layers. Also, the substrate may beformed, for example of bismaleimide-triazine (BT) resin. Therefore,although the invention has been described above using particularembodiments, many variations are possible within the scope of theclaims, as will be clear to the skilled reader, without departing fromthe spirit and scope of the invention.

1. A method for connecting a die assembly to a substrate in anintegrated circuit, the method comprising: attaching the die assembly tothe substrate, the die assembly comprising a first die and a second die;locating a flexible strip over the substrate and over the die assemblyafter attachment of the die assembly to the substrate, the flexiblestrip being separate from the die assembly and the substrate, theflexible strip carrying one or more conducting routing circuitsterminated at each end with a solder-on pad; and establishing electricalcontact between the first die of the die assembly and the second die ofthe die assembly and between the first and second dies of the dieassembly and the substrate via the one or more routing circuits carriedon the flexible strip.
 2. The method according to claim 1, whereinlocating the flexible strip over the substrate and the die assemblycomprises locating a polyimide strip.
 3. The method according to claim1, wherein establishing electrical contact comprises applying a heatingprocess to the flexible strip, the die assembly and the substrate tofuse solder on the solder-on pads with one or more bond pads on each dieof the die assembly and one or more bond fingers on the substrate. 4.The method according to claim 1, wherein an outer perimeter of theflexible strip is less than a perimeter of the substrate, but greaterthan a perimeter of the first die.
 5. A method for connecting a dieassembly to a substrate in an integrated circuit, the method comprising:attaching the die assembly to the substrate, the die assembly comprisinga first die and a second die; locating a flexible strip over thesubstrate and the die assembly, the flexible strip carrying one or moreconducting routing circuits terminated at each end with a solder-on pad;and establishing electrical contact between the first die of the dieassembly and the second die of the die assembly and between the firstand second dies of the die assembly and the substrate via the one ormore routing circuits carried on the flexible strip, whereinestablishing electrical contact comprises applying an ultrasonic processto the flexible strip, the die assembly and the substrate to fuse solderon the solder-on pads with one or more bond pads on each die of the dieassembly and one or more bond fingers on the substrate.
 6. A method forconnecting a die assembly to a substrate in an integrated circuit, themethod comprising: attaching the die assembly to the substrate, the dieassembly comprising a first die and a second die; locating a flexiblestrip over the substrate and the die assembly, the flexible stripcarrying one or more conducting routing circuits terminated at each endwith a solder-on pad; and establishing electrical contact between thefirst die of the die assembly and the second die of the die assembly andbetween the first and second dies of the die assembly and the substratevia the one or more routing circuits carried on the flexible strip,wherein locating the flexible strip over the substrate and the dieassembly comprises locating one or more apertures in the flexible stripon one or more projections on a holder retaining the substrate inposition.
 7. A method for connecting a die assembly to a substrate in anintegrated circuit, the method comprising: attaching the die assembly tothe substrate, the die assembly comprising a first die and a second die;locating a flexible strip over the substrate and the die assembly, theflexible strip carrying one or more conducting routing circuitsterminated at each end with a solder-on pad; and establishing electricalcontact between the first die of the die assembly and the second die ofthe die assembly and between the first and second dies of the dieassembly and the substrate via the one or more routing circuits carriedon the flexible strip; applying a template to the flexible strip afterlocating the flexible strip over the substrate and the die assembly toclamp the strip in position during the step of establishing electricalcontact.
 8. The method according to claim 7, wherein establishingelectrical contact comprises applying a heating process to the templateto punch one or more holes in the flexible strip through which one ormore retaining pins in the template extend to retain the flexible stripin position on the substrate and the die assembly.
 9. A method offorming a semiconductor device, the method comprising: placing a firstdie over a substrate, the substrate having substrate contacts on a firsttop surface of a first area, the first die having first bonding pads ona second top surface of a second area, the second area being smallerthan the first area; placing a second die over the first die, the seconddie having second bonding pads on a third top surface of a third area,the third area being smaller than the first area; placing a flexiblestrip over the second die; and electrically coupling the first die withthe second die using the flexible strip, wherein first routing circuitsdisposed within the flexible strip couple the first bonding pads withthe substrate contacts, and wherein second routing circuits disposedwithin the flexible strip couple the first bonding pads with the secondbonding pads, wherein the flexible strip is placed by positioning andretaining the flexible strip over the substrate, the first die and thesecond die using a removable template.
 10. The method according to claim9, wherein an outer perimeter of the flexible strip is less than aperimeter of the substrate, but greater than a perimeter of the firstdie.
 11. The method according to claim 9, wherein the second routingcircuits have first ends and opposite second ends such that the firstends contact the first bonding pads and the second ends contact thesecond bonding pads.
 12. The method according to claim 11, wherein thefirst ends and the second ends of the second routing circuits comprisesolder pads.
 13. The method according to claim 9, wherein the flexiblestrip, the first and the second dies are arranged about symmetricallyover the substrate.
 14. The method according to claim 9, wherein theflexible strip comprises polyimide material.
 15. The method according toclaim 9, wherein the flexible strip comprises a locating aperture. 16.The method according to claim 9, wherein the third area is smaller thanthe second area.
 17. The method according to claim 9, further comprisingretaining the flexible strip in position by extending one or more pinsprotruding from the removable template through one or more guide holesin the flexible strip.
 18. The method according to claim 9, wherein theremovable template comprises metal.
 19. A method of forming asemiconductor device, the method comprising: placing a die assemblycomprising a first die and a second die over a substrate, the substratehaving substrate contacts on a first top surface of a first area, thefirst die having first bonding pads on a second top surface of a secondarea, the second area being smaller than the first area, wherein thesecond die is over the substrate and the first die, wherein the seconddie comprises second bonding pads on a third top surface of a thirdarea, the third area being smaller than the first area; locating aflexible strip over the second die of the die assembly after placing thedie assembly to the substrate, the flexible strip having a fourth area;and electrically coupling the first die with the second die using theflexible strip, wherein first routing circuits disposed within theflexible strip couple the first bonding pads with the substratecontacts, and wherein second routing circuits disposed within theflexible strip couple the first bonding pads with the second bondingpads, wherein the die assembly comprising the first and the second diesis physically separate from the flexible strip before the electricalcoupling.
 20. The method according to claim 19, wherein an outerperimeter of the flexible strip is less than a perimeter of thesubstrate, but greater than a perimeter of the first die.
 21. The methodaccording to claim 19, wherein the second routing circuits have firstends and opposite second ends such that the first ends contact the firstbonding pads and the second ends contact the second bonding pads. 22.The method according to claim 19, further comprising forming the dieassembly before placing the die assembly over the substrate, whereinforming the die assembly comprises placing a first die over a substrateand attaching a second die over the first die.
 23. The method accordingto claim 19, wherein the fourth area being larger than the second area.